Plants, like any living organism are subjected to numerous environmental stresses which are both biotic and abiotic. Biotic stresses include pests such as insects, arachnids, and nematodes, and pathogens such as bacteria, viruses and mycoplasms. Abiotic stresses would include drought and extremes in temperature. Each year these stresses result in billions of dollars worth of loss as a result of damaged or diminished crop production. Thus, control of the effects of such stresses on valued plants is one of man's major concerns.
Since 1945, control efforts to protect plants against such stress factors have utilized principally synthetic toxic chemicals (pesticides). Annual usage of pesticides has increased to over 544 million kilograms. Pesticides, however, are expensive to bring to market and consequently expensive to use. Furthermore, because of their persistance in the environment, they present a continually growing health risk to animal and human life.
The economic and health risk cost of pesticide use has led to an ever-increasing emphasis on alternative strategies of plant protection. One such alternative strategy has been to search for methods to enhance the plants own defense mechanisms.
It has been known for some time that certain stressful stimuli will increase a plant's resistance to pathogens. In 1940, Muller and Borges discovered phytoalexins. Muller, K. O. and Borges, H. Arb. Biol. Reichsanst Land-u Forstwiss. 23 189-231 (1940). The discovery of phytoalexins provided the biochemical explanation for what had been observed to be an inducible defense response. Subsequently, it has been shown that exposure to a variety of biotic or abiotic stresses (exo-elicitors) will cause the plant to synthesize and accumulate phytoalexins. These phytoalexins display antifeedant and antibiotic properties which are protective to the plant. They have been shown to be toxic to fungi, bacteria, higher-plant cells, and also animal cells. J. Ebel, Phytoalexin Synthesis: The Biochemical Analysis of the Inductive Process, 24 Ann. Rev. Phytopathol, 235-64, 1986. These exo-elicitors may also induce other chemical defense mechanisms besides phytoalexins, e.g., protease inhibitors and hormone mimics.
Biotic exo-elicitors which have been studied to date include: Phytophthora megasperma var. sojae (a fungus) (Klarman, W. L. Netherlands Jour. Plant Pathol. 74: 171-175 (1968); Chamberlain, D. W. and J. D. Paxton, Phytopathology 58: 1349-1350, (1968); Meloidogyne incognita (a nematode) (Kaplan, D. T., N. T. Keen and I. J. Thomason. Physiol Plant Pathol. 16: 309-318, 1980); Pseudomonas syringae pv. glycinea (a bacterium) (Holliday, M. J., N. T. Keen and M. Long. Physiol. Plant Pathol. 18: 279-287, 981); several species of insects (Kogan, M., and J. Paxton, in: P. A. Hedin (ed.), Plant Resistance to Insects, Amer. Chem. Soc., Wash., D.C. 1983); Tetranychus urticae (a mite) (Hildebrand, D. F., J. G. Rodriguez, G. C. Brown, K. T. Luu and C. S. Volden. Jour. Econ. Entomol. 79: 1459-1465, 1986); and Epilachna varivestis (an insect) (Chiang, H. S., D. M. Norris, A. Ciepiela, P. Shapiro and A. Oosterwyk. Jour. Chem. Ecol. 13: 741-749, 1987).
Although such biotic exo-elicitors have been shown experimentally to increase plant resistance, they may be themselves pests or pathogens of plants. Further, large scale production of biotic exo-elicitors that display uniform activity in a quantity necessary for practical use would be difficult and costly under the best of circumstances. Thus, at present, biotic exo-elicitors do not appear to be a satisfactory alternative to toxic pesticides.
Abiotic exo-elicitors have been identified among: fungicides and fungicidal decomposition products (Reilly, J. J. and W. L. Klarman. Phytopathology 62: 1113-1115, 1972); maneb, ethylenediamine, polyethylene (thiocarbamoyl) monosulfide (PTM) and benomyl are representative of such fungicides; ultraviolet irradiation was active in soybean (Bridge, M. A. and W. L. Klarman. Phytopathology 63: 606-609, 1973); mercuric chloride (Moesta, P., and H. Grisebach. Nature 286: 710-711, 1980); acifluorfen and oxyfluorfen herbicides (Komives, T., and J. E. Casida. Jour. Agric. Food Chem. 31: 751-755, 1983); dithiothreitol (DTT), N-ethylmaleimide (NEM), p-hydroxymercuribenzoate (PHMB) and p-chloromercuribenzenesulfonic acid (PMBS) (Stoessel, P. Planta 160: 314-319, 1984); and a glucan molecule (Grisebach, H., H. Boerner, M. L. Hagman, M. G. Hahn, J. Leube and P. Moesta. UCLA Symp. Mol. Cell Biol., Ser. 22: 275-290, 1985).
Unfortunately, these experimental abiotic exo-elicitors persist in the environment and are toxic to living organisms--both plant and animal. Thus, while useful for study, they do not avoid the problems already presented by the toxic pesticides.